1. Field of the Invention
The present invention relates to investment casting using stereolithography, and, more particularly, to a resin composition for eliminating breakage of a ceramic mold coating the resin composition during processing.
2. Description of Related Art
Investment casting is an industrial process which employs a disposable pattern (specimen or model) that is used to produce a mold in which parts can be cast. The pattern is made by injecting wax or plastic into a pattern die which has been manufactured by machining processes.
The pattern produced in the pattern die, which corresponds to the three-dimensional specimen used herein, is melted or burned out of the mold in which parts are later cast. Since the pattern possesses the shape of the required finished part, the parts which are later cast have the required shape.
The mold is built up around the pattern by a well-known process, typically coating the pattern with several coats of a ceramic slurry, with a drying step between each coating step. The pattern which is invested in the ceramic is then placed into a furnace or autoclave, causing the wax or plastic pattern to be melted or burned out of the resulting mold.
Removal of the pattern leaves a cavity in the mold, corresponding in shape and dimension to the final part. Molten material is then introduced into the mold and solidified by cooling. After solidification, the ceramic mold is broken away to release the finished part.
In an attempt to provide a pattern material that would not break the mold due to thermal expansion during burnout, a pattern produced by stereolithography was disclosed in U.S. Pat. No. 4,844,144, issued Jul. 4, 1989. The stereolithography composition comprises an ethylenically unsaturated liquid material in admixture with an easily heat-softenable thermoplastic material, such as an oligomer of low molecular weight which is inert with respect to the unsaturated material, which weakens the pattern when heated within the mold. The modified liquid composition cures in the presence of light to form a cross-linked polymeric matrix having interstices which are filled with the thermoplastic oligomer, and this oligomer functions to prevent the mold cracking by weakening the pattern before final burnout of the resin.
However, use of the inert diluent disclosed causes pattern weight losses of only 4 to 6% at 200.degree. C. and 13 to 21% at 300.degree. C. No weight loss at 100.degree. C. was observed, and no indication of the temperature at which shrinking and cracking occurs is given.
Thus, a need remains to provide a resin composition for investment casting that does not cause breakage of the mold during burnout of the pattern and that leaves no residue in the mold after burnout.